A requirement exists to train personnel for low light level night viewing conditions both with the naked eye and also with devices such as Night Vision Goggles (NVGs). NVGs, which are optical devices with electronic light amplification capability, are widely used in a variety of applications including various types of military operations. The training of personnel to efficiently use NVGs and to see unaided under low level light conditions is carried out in several ways. Included among these is the use of projector slides having films with photographic, artwork drawings or computer generated images of various types of information such as possible battle scenes. The film image is projected onto a surface, such as a viewing screen, at light levels encountered during the night and the trainee views the projected image with unaided night vision or though the NVG.
Slides used with a light projector during low light level training with unaided night vision or with NVGs must satisfy a variety of requirements. Included among these is that the amount of light coming from the projector must be reduced by a factor of about 1,000,000 from that of projector light output in the more usual situation of a slide projector producing light under normal ambient light conditions. If the light level is not reduced, then the NVG electron-optics will be saturated since the NVG is designed to operate at extremely low level light conditions.
At the very low level of light required in an unaided night vision or NVG application, the control of light fluctuations on the slide film is difficult and becomes critical. Also, the spectrum of the light projected onto the slide may have to be altered from the normal human visible light range spectrum to be compatible with the spectral sensitivity of the NVG. In addition, the slide film can absorb heat from the projected light which can cause distortions in the slide film resulting in a distortion of the projected image.